Bronchiolitis is the #1 cause of infant hospitalization in the USA. Small cohort studies (n<210) suggest that 40-50% of hospitalized infants with bronchiolitis will develop childhood asthma. Unfortunately, it remains unclear which infants will develop asthma and this knowledge gap has hindered primary prevention efforts. The 35th Multicenter Airway Research Collaboration (MARC-35) study (U01 AI-87881; Camargo, PI) is a 17-center prospective cohort study that will complete enrollment of ~940 hospitalized infants with bronchiolitis (80% ward, 20% intensive care unit) in April 2014. In this diverse U.S. cohort (~52% African-American or Hispanic), site investigators have collected nasopharyngeal and blood samples (including DNA); extensive interview and survey data; and medical records from primary care, emergency department, and inpatient settings. Follow-up data include biannual parent interviews and annual review of medical records (~91% follow-up to date). For timing reasons, the primary outcome of the 5- year U01 grant is recurrent wheezing by age 3 years. However, all participants were consented for follow-up to age 6 years to permit ascertainment of asthma (as defined by doctor diagnosis, plus either asthma medication use or symptoms in past year); incident asthma at age 5 years is the primary outcome of this R01 ancillary application. The Specific Aims address two risk factors for childhood asthma: 1) specific IgE, which preliminary data show in ~20% of MARC-35 infants; and 2) rhinovirus type C, a still-undefined subset (likely half) of the ~21% of infants with rhinovirus bronchiolitis. Aim 3 examines the potential interaction between these two factors and incident asthma. Preliminary data (n=745) already show a strong interaction between infant specific-IgE and rhinovirus (not yet typed) and two available outcomes: risk of recurrent wheeze by age 12 months and use of inhaled corticosteroids by age 18 months (both Pinteraction<0.01). The R01 would provide funds to examine IgE sensitization longitudinally by testing serum specific IgE at age 42 months (3.5 years), rhinovirus typing by partial sequencing for rhinovirus samples from the index hospitalization, and continued follow-up with biannual telephone calls and annual chart reviews through age 5 years; these activities are not funded by the U01 grant. The application is time sensitive because of the new 42-month exam and follow-up after age 3 years. The study has >80% power for all aims. The investigators are NIH-funded researchers with international expertise in the field. The study advances research on the primary prevention of asthma, and matches well with the 2009 NIH strategic plan for pediatric respiratory research.